1. Field of the Invention
The present invention relates to a method for manufacturing a compound semiconductor having at least one layer, which is a carbon-doped p-type semiconductor epitaxial layer produced by a metalorganic vapor phase growth process.
2. Description of the Related Art
As a doping method for manufacturing a p-type semiconductor epitaxial layer of a carbon-doped GaAs, AlGaAs, or InGaAs having a p-type conduction to be manufactured by a metalorganic vapor phase growth process (hereinafter, may be referred to as MOVPE process), techniques using carbon tetrachloride and carbon tetrabromide as a carbon source are known. Use of the above carbon sources results in a good incorporation efficiency of carbon into a p-type semiconductor epitaxial layer. This can provide a high carrier concentration of GaAs, AlGaAs, or InGaAs in the epitaxial layer having a p-type conduction. The high carrier concentration carbon-doped p-type GaAs and p-type InGaAs thus manufactured are used as, for example, a base layer of a Hetero Bipolar Transistor (HBT). The carbon in GaAs has a small diffusion constant in comparison with other p-type impurities such as Be and Zn, which enables the growth of a base layer excellent in reliability. The characteristic of the base layer is one of the factors that exerts a large influence on the HBT device characteristics.
However, in order to dope a p-type semiconductor epitaxial layer of GaAs, AlGaAs, or InGaAs with carbon to a high concentration, by using carbon tetrabromide as a carbon source, which has a melting point of 90.degree. C., and is a solid at room temperature, the doping needs to be carried out by sublimation. However, as carbon tetrabromide has a low vapor pressure of 0.6 mmHg/23.degree. C., although it is still a high efficiency carbon source, it is instable in vapor pressure, and it may be difficult to realize sufficient vapor pressure. Also, in the future, in the case where an increase in diameter of an epitaxial substrate, or an increase in size of a device for the purpose of productivity improvement is required, the supply may sometimes become insufficient for obtaining a semiconductor epitaxial substrate having a high carrier concentration.
It is also known that, in the growth of a high carrier concentration semiconductor epitaxial layer having a p-type conduction by a MOVPE process, an increase in supply of carbon tetrachloride or carbon tetrabromide, or an increase in growth temperature for enhancing the crystallinity of the epitaxial film results in the formation of hydrogen chloride or hydrogen bromide in the reaction process; this causes etching to proceed, and results in a difficulty in control of the epitaxial film thickness (J. Appl. Phys. 72, 2063 (1992), Japanese Laid-Open Patent Publication No. Hei 9-219370). Especially, the etching amount is large in the case where carbon tetrachloride is used as a carbon source, and hence it is not preferable.
An object of the present invention is to provide a method for manufacturing a compound semiconductor having at least one layer, which is a carbon-doped p-type semiconductor epitaxial layer produced by a metalorganic vapor phase growth process, where the etching amount during growth is relatively small, and carbon can be added to a high concentration even with a large MOVPE apparatus.